test(omchem): improve automated test coverage for OMChem unit operations and Flowsheet simulation - #142

This PR significantly expands automated unit test coverage for the OMChem package and its core simulation orchestrator (Flowsheet) and achieves 100% test coverage across all OMChem runtime modules.

Key Improvements

1. OMChem Unit Operations (tests/test_omchem_models.py):

   • Added comprehensive tests for DistCol and ShortcutColumn covering OM_Flowsheet_Init and OM_Flowsheet_Eqn across diverse modes (reflux ratio vs. temperature/side-draw,
     single/multiple feeds, total vs. partial condenser, and energy streams Qcond/Qreb).
   • Covered AdiabaticCompressor and AdiabaticExpander across all supported modes (pressInc, outP, reqPow, pressDrop, genPow).
   • Added complete coverage for CompSep, Heater, Cooler, Pump, Valve, Flash, Mixer, Splitter, and EngStm parameter getters/setters, getname(), modesList(),
     and energy flow (enFlo) equation handling.

2. Flowsheet Simulation Lifecycle (tests/test_flowsheet.py):

   • Covered Sequential-Modular (SM) simulation lifecycle (simulate_SM and send_for_simulation_SM), validating topological sort resolution across all 4 branching conditions,
     stream property updates (stm.Prop), and multi-unit CSV aggregation into FlowsheetSEQ.csv.
   • Covered Equation-oriented (EQN) execution with mock OpenModelica (omc) processes, package.order synchronization, and cleanup of stray .mo files.
   • Tested Windows and Linux dynamic path resolution for omc, startup flags, error decoder fallbacks, and CSV result parsing edge cases.